I tried changing the names already, to no avail. I can get the mesh to load, but it only appears in first person, and isn't where it should be, regardless of any changes I make to position, rotation, or scale. I'll try making a separate anim_base.1st.nif file and see if that does anything.

With a lot of help from DestinedToDie, I managed to get it so there's a working model that doesn't depend on Bethesda's rig, using Moonboy as a basis. I imported Moonboy's NIFs into Blender and they re-export fairly well, and I was able to replace Moonboy's extant body parts with parts of my own. For some reason, the non-Moonboy body parts don't animate (which is weird because they work just fine with the vanilla rig), so that's next on the to-do list. I also need to adjust proportions a bit.

The bones have now been proportioned and positioned properly, and attachment points for clavicles, weapon, hair, and shield have been added. I'm also learning a lot about the logic behind certain things, like why all of the *_skins.nif files include the hands and torsos. After reverse engineering all of this, I'm bound to be one of the foremost experts in the subject.

I want to see if it'll support IK solvers, and open-close bones for the hands, but those will probably have to wait as I hope to be busy with other things for a few days. I expect that animations will have to be redone basically from scratch.

P.S. I'd like to apologize if I've been a pain in the ass to anyone during this process.